In a CDMA communications system a plurality of user communication signals can be transmitted within, i.e., share, a same portion of the frequency spectrum. This is accomplished by providing a plurality of different pseudonoise (pn) binary code sequences (e.g., one for each user) that modulate a carrier, thereby "spreading" the spectrum of the resulting waveform. In a given receiver all of the user signals are received, and one is selected by applying an assigned one of the pn binary code sequences to a correlator to extract only the signal energy intended for the receiver, thereby "despreading" the received CDMA transmission. All other (uncorrelated) user transmissions appear as noise.
One type of CDMA communication system is specified by a document referred to as EIA/TIA/IS-95. The system as specified uses a plurality of base stations that establish and maintain bidirectional direct-sequence (DS) CDMA links with a plurality of mobile stations (e.g., cellular telephones). One feature of the IS-95 system is the presence of a pilot channel that is transmitted by each base station.
The pilot channel is an unmodulated, direct-sequence spread spectrum signal that is transmitted continuously by each CDMA base station. The pilot channel allows a mobile station to acquire the timing of the Forward CDMA channel (i.e., from the base station to the mobile station), provides a phase reference for coherent demodulation, and provides a reference for signal strength comparisons between base stations for determining when to handoff. The pilot pn sequence is defined as a pair of modified maximal length PN sequences with period 2.sup.15 that are used to spread the Forward CDMA channel and the Reverse CDMA channel. Different base stations are identified by different pilot PN sequence offsets. A pilot pn sequence offset index is defined to be in units of 64 pn chips, relative to a zero offset pilot pn sequence. A pn chip is defined as one bit in the pn sequence. The pilot strength is defined as the ratio of received pilot energy to overall received energy.
Walsh functions are a class of 2.sup.N time orthogonal binary functions that are used to establish orthogonality between the different pn binary code sequences used by the pilot and user channels.
The use of the pilot channels, while providing certain advantages in a CDMA system intended for use with mobile stations, may present disadvantages as well, particularly in systems where the user transceivers are fixed as opposed to mobile. For example, the pilot channels consume some amount of the available pn code sequences and signal energy, all of which could be otherwise allocated to the users of the system.
Also, in many detection approaches synchronization to the pn code timing must be achieved before carrier phase-lock can occur. In this case a non-coherent detection algorithm must be employed. Generally, non-coherent detectors rely on energy detection within a fixed bandwidth, as a range of code timing cells are searched. Upon locating the correct code timing, the detector energy level rises above a predetermined threshold level. A bit-sync loop then takes over to obtain the finer-resolution bit timing.
However, standard acquisition approaches are known to fail when the number of users becomes large. This is due to the fact that the noise power becomes comparable to the signal power when the user of a synchronous type of CDMA system is not synchronized. As a result, it becomes very difficult for the user's receiver to distinguish the correct pn timing phase from the incorrect phases resulting from the increased noise.
As can be appreciated, the acquisition technique is an important aspect of the receiver, in that its operation impacts the overall speed at which synchronization to the forward link occurs. If the time required to synchronize the user's receiver becomes excessive, the delay may be considered as objectional by the user.